1. Field of the Invention
The present invention relates to a method for purifying an etching solution consisting of an aqueous phosphoric acid solution which has been used for etching of a silicon nitride film formed on a semiconductor wafer. In particular, it relates to a method for removing a silicon compound contained as an etching reaction product in an etching solution.
2. Discussion of Background
In a wet etching process for manufacturing semiconductor devices, etching is generally conducted while filtering an etching solution to remove impurities such as contaminative particles in the etching solution and recirculating the filtered etching solution so as to maintain the etching solution clean. When reaction products resulting from etching accumulatively increase in the etching solution to precipitate, the precipitates are also removed in the filtration process. In this filtration process, precipitation of reaction products resulting from etching may be urged by means of cooling or the like to effect filtration.
Also in etching of a silicon nitride film with a heated aqueous phosphoric acid solution, as in the above process, a method for filtering and recirculating an etching solution has been known (see U.S. Pat. No. 4,980,017). When silicon compounds as reaction products resulting from etching precipitate in the etching solution, the precipitates may be filtered off together with other impurities to purify the etching solution. However, precipitated silicon compounds cause degradation of an object to be etched. Accordingly, when the concentration of the reaction products resulting from etching in the etching solution becomes high, the etching solution is generally replaced to prevent a semiconductor wafer from being adversely affected by the precipitates.
As mentioned above, in customary etching methods, an etching solution is required to be replaced periodically. Such repeated replacements are bothersome and economically disadvantageous because of the use of the chemical agent in an increased amount. In the methods comprising removal of precipitated reaction products resulting from etching by filtration, a filter is required to be washed periodically to prevent clogging of the filter, thereby hindering efficient operation of an etching plant. In the process for etching a silicon nitride film with a heated aqueous phosphoric acid solution, the etching solution is particularly corrosive and also has a high temperature, thereby leading to troublesome cleaning operation of a filter.
In LOCOS method or the like which is used for the production of an MOS type LSI, selective etching of a silicon nitride film between silicon dioxide film/silicon nitride film is carried out. In the etching, the selection ratio therebetween depends upon the concentration of a silicon compound(s) dissolved in the etching solution as a reaction product(s) resulting from the etching as well as upon the concentration of phosphoric acid.
Variation in the selection ratio due to the change of the concentration of phosphoric acid can substantially be prevented by conducting etching while adding water in an amount corresponding to the amount of water evaporating out of an etching bath. However, variation in the selection ratio due to change in the concentration of the dissolved silicone compound(s) cannot be avoided unless the etching solution is frequently replaced.